Cryogenic SiGe Hetero-Junction Bipolar Transistors From Standard Technologies For Low Noise FLL

Dynamic range of Superconducting QUantum Interference Devices (SQUID) can be considerably increased by using a Flux-Locked-Loop (FLL). The FLL noise is mainly determined by the amplifier noise used in the feedback loop. The use of a very low noise amplifier working at cryogenic temperature can lead to reach the limit of SQUID intrinsic noise (typically few 10
^-6
Φ
₀
/√Hz). To realise the amplifier of the FLL, the use of standard technologies allows to design an ASIC including several amplifiers which are necessary for the read-out of a SQUID array. We present here some experimental results showing that two different standard BiCMOS SiGe technologies can operate at 4K. DC and low frequency measurements were carried out on two SiGe Hetero-Junction Bipolar Transistors (HBT) of 0.8 μm and 0.35 μm AMS BiCMOS SiGe technologies. We report that for both SiGe HBTs, the transconductance gm increases at cryogenic temperatures making possible the use of such devices down to 4K for low noise voltage amplification. However, the current gain β remains highly dependant on the fabrication process. Thus, we found that the current gain of the 0.8 μm technology HBTs decreases at low temperature whereas it increases for the 0.35 μm technology. The discussion is centred on the effects which could explain the behaviours of β and gm at temperature down to 4K. Noise measurements are also presented confirming the existence of an effective temperature which increases the noise expected at 4K. The presented results can lead to the realisation of cryogenic ASICs for low noise FLL readout of bolometers arrays dedicated to Cosmic Microwave Background (CMB) observations.